A steam generator can be designed in accordance with different layout principles. In a continuous steam generator the heating of a number of steam generator pipes which together form the gas-tight enclosing wall of the combustion chamber leads to a complete evaporation of a flow medium in the steam generator pipes in one operation. The flow medium—usually water—is fed after its evaporation to the superheater pipes downstream from the steam generator pipes and is superheated there.
A continuous steam generator, by contrast with a natural circulation steam generator, is not subject to any pressure limiting, so that it can be designed for fresh steam pressures far above the critical pressure of water (Pcrit=221 bar)—with no distinction being possible between the water and stream phase and thereby no phase separation being possible either. A higher fresh steam pressure facilitates a greater efficiency and thereby lower CO2 emissions in a fossil-fueled power station.
For steam generators with a vertical gas draft the steam generator pipes are generally connected to each other via fins The encircling wall is thus formed from a number of approximately parallel steam generator pipes which are connected to each other via fins and welded so as to be gas-tight. The steam generator pipes of the steam generator can be arranged vertically or in a spiral form and thereby inclined.
Funnel-shaped side walls of the combustion chamber are usually arranged at the lower end of the gas draft pipe, said walls being formed to allow the uncomplicated removal of ash occurring during the combustion process. In this case the combustion chamber wall is generally formed from vertical steam generator pipes and fins. In the lower section, in the area of the funnel, the steam generator pipes usually also run on in the manner of vertical pipework in the same direction as in their upper section forming the combustion chamber wall. The parallel pipes enter the funnel in this case via entry collectors and in the continuation of the parallel pipes form the combustion chamber.
During the operation of a continuous steam generator, the heat generated during the combustion of a combustion gas within the combustion chamber is entered both directly via the walls of the steam generator pipes and also via the fins into the flow medium flowing through the steam generator pipes. In this case the heating steam generator pipe determines the weight of the column of water in the relevant pipe. Since the throughflow of flow medium through a steam generator pipe and thereby the output temperature of the flow medium depends on the pressure of the column of water in the corresponding pipe, the output temperature through a steam generator pipe will be decisively influenced by the heating of the corresponding steam-generator pipe.
If the steam generator pipes are heated by different degrees, the result is thus different output temperatures of the flow medium. Under some circumstances—especially during startup processes and at low loads—such temperature differences can reach a high value, in which impermissibly high loads are imposed on materials.
For steam-generator pipes running in the combustion chamber wall and in the area of the funnel-shaped side walls, there are a number of steam generator pipes and the associated fins in the area of the funnel-shaped side walls, namely those which, for a rectangular cross-section of the combustion chamber lie in the area of the four corners, which are shorter than those which form the tip of the funnel-shaped side walls. Because of their different length the steam generator pipes and the fins are thus subjected to different levels of heating. There is thus the danger, on account of the different levels of heating of the steam generator pipes in the area of the funnel-shaped side walls, of impermissibly high temperature differences of the flow medium leaving the individual steam-generator pipes arising.